Bar codes and other machine-readable symbols, as well as bar codes on many products, badges, and the like, are produced by on-demand printers. As depicted in FIG. 1, on-demand bar code printers employ a print assembly such as a thermal printhead 10 to print the bar codes on a media. As depicted in FIG. 2, the printhead 10 comprises a series of adjacent thermal elements 12 that print the pixels or dots on the surface of the media. The media can have a thermally activated surface which turns dark when subjected to heat or there can be a "ribbon" of "ink" which is transferred to and fused to the surface of the media by heat. In either case, the printer creates a dot 14, an example of which is depicted in FIG. 3, although other shapes for such dots are common.
As depicted in FIG. 4, various thermal elements 12 in the printhead 10 are selectively heated as the media passes beneath it, and a series of bars such as wide bars 16 and 18, separated by spaces such as narrow space 20, are created. The bars 16, 18 and space 20 are created in pre-established patterns as symbol characters which define the desired information. The bars 16, 18 and space 20 form part of a symbol 21, shown in FIG. 5A. FIG. 5A shows an exemplary laminated bar code symbol 21. The symbol 21 is a Code 39 symbol, and thus contains only elements having two widths, wide bars and spaces and narrow bars and spaces.
Bar code symbols are often laminated or positioned so that a reflective surface is placed thereover, so as to protect the underlying symbol. An "over-laminate" is typically a coating or material adhered to the scanning surface of a bar code symbol. The opacity, gloss (optical effects) and smoothness (physical effects) of the over-laminate affect the decodability of underlying symbols which is a measure of the "goodness" or accuracy of the symbol when received by the scanner. A laser beam or other light source passing over a laminated bar code symbol will cause some of the laser light to be scattered or absorbed by the laminate material. In other words, light reflects off the laminate or overlying transparent surface, producing specular reflection. As a result, the modulation print quality parameter of the symbol is degraded or reduced. The "modulation" of a symbol is the ratio of a minimum edge contrast to the overall symbol contrast. The "edge contrast" is the difference between the reflectance of a space and the reflectance of an adjoining, bar, where the minimum edge contrast is the smallest edge contrast in a profile produced from a scan of the symbol. The "symbol contrast" is the difference between the largest and smallest reflectances in a scan reflectance profile. The modulation in laminated bar code symbols is reduced particularly where spaces are surrounded by wide bars. These spaces appear duller than the quiet zones which surround the symbol, as well as less bright or more gray than other spaces within the symbol.
For example, FIG. 6 shows an exemplary profile 22 as produced by a wand scanner from the laminated bar code symbol 21. Also shown in FIG. 6, below the profile 22, is a waveform 24 showing a thresholding or squaring off of the profile 22. Below the waveform 24, FIG. 6 shows printing, quality parameters for the profile 22, and space and bar counts derived from the profile. Importantly, the modulation of the profile 22 is only 56%, thereby having only a grade C rating. Such a low modulation is due, in part, to the reflectance of the laser beam from the laminate such as is produced by the relatively low peak 20' produced by the wide space 20 in the symbol 21. The surrounding wide bars 16 and 18 (FIG. 4) produce sufficiently low valleys 16' and 18' in the profile 22, as shown in greater detail in FIG. 7, which shows an enlarged view of the right-hand portion of the profile 22. A similar, relatively low peak 23 produced by a space in the symbol 21 is also bordered by two wide bars. The low peaks 20' and 23, relative to other peaks produced by other spaces, and the quiet zone, result in the low modulation value for the profile 22. As a result of the low modulation, the profile 22 has only an overall print quality grade of C.